1. Field of the Invention
This invention relates to a process for manufacturing hydrate salt-based granulate-shaped thermal energy accumulators with an hermetic --i.e. water- and gas-proof--coating perfectly adhering to their surface, to the accumulators obtained by this process and to their use.
Hydrate salts--such as sodium, calcium and magnesium sulphates, phosphates, chlorides and nitrates in various degrees of hydration--are a particularly interesting material for thermal energy accumulation. In this respect they possess a high latent heat of hydration over a very wide temperature range, for example between -20.degree. C. and +100.degree. C., and are therefore very suitable for many applications, such as in solar energy plants and generally in all those plants in which a high thermal energy absorption/accumulation capacity is required at constant temperatures both less than and exceeding 0.degree. C.
2. Description of the Prior Art
The use of these salts however has not, at least up to recent times, undergone adequate development as the aforesaid advantageous characteristics have always been accompanied by the serious drawback that after only a few solidification and fusion (i.e. hydration and dehydration) cycles, the salts permanently separated from their crystallization water, to thus loose all capacity for thermal accumulation in the form of latent heat.
After lengthy studies, the present Applicant discovered that this drawback could be obviated by enclosing hydrate salts in elementary cells having an internal volume of less than 100 cc and their three dimensions all of the order of centimeters. This discovery lies at the base of patent No. EU-A-0076897.
This patent also describes some processes for manufacturing these elementary cells. In these processes, the anhydrous salt is hydrated and melted in a suitable reactor, and then cast into plastics cells of different shapes. These cells are then sealed.
The aforesaid teachings have finally allowed manufacture of hydrate salts accumulators which are industrially useful, i.e. able to withstand a very large number of cycles without any permanent separation taking place between water of crystallization and salt inside the element, thus ensuring a practically unlimited useful life of the same.
The maufacturing processes shown in the specification No. EU-A-0076897 have however proved to be accompanied by certain problems. Firstly, the automation of these processes requires careful setting of the equipment for the preparation of the melted hydrate salt and its injection into the cells. Secondly, the cell sealing process, when filling is complete, must be carefully controlled in order to eliminate any air from each cell, and also because even a minimum weld imperfection--caused for example by hydrate salt residues in the welding region--leads to rapid deactivation of the cell by evaporation of the crystallization water there contained. Consequently it is not possible to obtain the high productivity required by the increasing market interest, and then the production cost of cells manufactured by these processes remains still too high and does not favour large-scale adoption.